Selfbonded nonwoven fabrics

ABSTRACT

A selfbonded nonwoven fabric comprising at least 70 weight percent of sheath/core heterofilaments having a core of isotactic polypropylene in which the sheaths are high density polyethylene in an amount of 5 to 30 weight percent of the heterofilaments, and method of preparation. The fabrics have outstanding strength, fatigue resistance, and tear resistance and are eminently suitable for civil engineering applications such as unpaved road underlay material.

This is a continuation-in-part application of application Ser. No.776,565, filed Mar. 11, 1977, now abandoned.

BACKGROUND OF THE INVENTION

(i) Field of the Invention

This invention relates generally to porous nonwoven fabrics formed fromselfbonded heterofilaments. More particularly it relates to such fabricsin which the heterofilament has a core of isotactic polypropylene and asheath; the sheath is high density polyethylene; and the sheathcomprises from 5 to 30 weight percent of the heterofilament.

(ii) Description of the Prior Art

It is well known that batts or webs consisting solely of randomly laidthermoplastic homofilaments having essentially identical propertiescannot be area-selfbonded commercially by application of heat andpressure to yield a product having both high grab strength and Elmendorftear strength, on account of extreme criticality of the bondingconditions. For example, du Pont's U.S. Pat. No. 3,546,062 teaches anacceptable bonding temperature range of only a fraction of a degreeCentigrade for polypropylene fibers, other conditions being keptconstant, and that overbonding results in a cardboard like product. Theprior art further shows that the operating range of permissible bondingtemperature is greatly increased by deliberately introducing variabilityinto the raw materials and/or processing conditions in many differentways. For example, U.S. Pat. No. 3,231,650, Example 1, teaches that thestrength of selfbonded nonwoven webs of drawn high density polyethylenehomofilaments having a softening point of 260° F. may be increased by afactor of 10, as measured by manual application of tension to theselfbonded web, by soaking the drawn fibers in hydrocarbon oil for 15minutes prior to bonding at 83.3 p.s.i. for 5 minutes at 250° F. Thepatent does not discuss how the hydrocarbon oil causes the increase instrength, but it presumably preferentially plasticizes the amorphousportions of the highly crystalline filaments. Another method ofintroducing variability is that of using minor quantities of alower-melting "binder" fiber to effectuate the desired amount ofbonding. The binder fiber may be spun as separate filaments from thestrength-providing fiber (as in du Pont's U.S. Pat. No. 3,546,062 andKuraray's U.S. Pat. No. 3,914,497) or it may be co-spun with some or allof the strength-providing fiber to give heterofilaments (as in ICI U.S.Pat. Nos. 3,511,747; 3,423,266; and 3,595,731; and ICI's U.K. Patents1,157,437 and 1,073,181; Chisso's German Offenlegungsschrift 2,358,484and Mitsubishi Rayon's Japanese Pat. No. 50-4767; and in "MechanicalBehavior and Bonding In Nonwovens", a dissertation presented toPrinceton University by C. J. Shimalla, June 1974).

The ICI patents such as U.S. Pat. Nos. 3,511,747 and 3,595,731, disclosethe preamble of applicants' claimed invention, including a generaldisclosure of polyethylene as the potentially adhesive component of theheterofilament. However, there are no examples directed to high densitypolyethylene.

Prior art relating to the use of high density polyethylene in filamentsincludes the following.

Shimalla studied in depth the factors affecting the properties ofnonwoven fabrics formed from card webs of mixtures of homofilaments andheterofilaments in which the binder was linear polyethylene in the formof a 50/50 bilateral bicomponent fiber, the conjugate half beingisotactic polypropylene, and the fabrics contained up to 50 weightpercent of linear polyethylene. Shimalla also points out thatsheath/core filaments provide about four times as many bond points as50/50 bilateral filaments and found that the relative distance betweenbonds was one of the most critical parameters in filtration applicationsof these nonwovens. Shimalla further analyses principles of bonding insemicrystalline polymers and points out that the bond strength isdetermined by a complex combination of physical, chemical and mechanicalproperties of the materials making up the bond, by the conditions underwhich the bond is formed, and by the consequent bulk and local stresses.Shimalla's requirements for practical adhesive bonding are: (1) ensurethat no weak boundary layer is present on the substrate; (2) use anadhesive having a surface tension less than the critical surface tensionof the substrate; (3) form extensive interfacial contacts by the choiceof bonding conditions; and (4) set (cure or crystallize) the adhesive tomaintain interfacial contacts, prevent frozen stresses, and eliminateweak boundary layers.

U.S. Pat. No. 3,914,497 teaches that high density polyethylenehomofilaments used as 10% binder fiber for polypropylene fibers gives abonded fabric having very low tensile strength (see U.S. Pat. No.3,914,497 Comparative Example 8).

U.S. Pat. No. 3,620,892 discloses fabrics of fused heterofilamentsconsisting of minute fibrils which may be polypropylene andpolyethylene. No distinction is made between high density polyethyleneand low density polyethylene.

U.S. Pat. No. 3,760,046, column 7, teaches the sintering of fabrics withlow thread counts in which the filaments have a low density polyethylenesheath and a core of either polypropylene or high density polyethylene.

U.S. Pat. No. 2,861,319 teaches that low density filaments containingnon-continuous voids may be prepared by drawing sheath/core filaments inwhich the core has less extensibility than the sheath, and does not showthe use of a high density polyethylene sheath with an isotacticpolypropylene core.

U.S. Pat. No. 3,998,988 relates to fibrous material having particulatematerial studded in the surfaces of the filaments. It discloses afibrous adsorptive material in the form of tow, web, fabric, sheet, ballor flock consisting of a sheath/core conjugate fiber of a high meltingcore component and a low melting sheath component, with finely dividedparticles of an absorbent embedded in the surface of the low meltingsheath component. It further teaches that the sheath has a melting pointat least 40° C. lower, and preferably 50° C. lower, than the meltingpoint of the core, and polyethylene may be the sheath. Example 20 of thepatent discloses fibrous beads formed by heating a tumbled mixture of55% sheath/core drawn filaments (5 mm long with the core beingpolypropylene and the sheath being polyethylene having a melting pointof 132° C.) with 45% of particulate absorbent material, whereby theabsorbing agent particles are melt adhered to filament surfaces andfixed among filaments to form a bead-like fibrous absorbent. Thepresence of particulate material between two frozen surfaces would beexpected to significantly weaken the bond strength.

Chisso's German Offenlegungsschrift No. 2,358,484 discloses nonwovenfabrics comprising crimped bicomponent heterofilaments of polypropyleneand high density polyethylene. It does not exemplify sheath/coreheterofilaments, but rather rod/crescent side/side heterofilamentsapproaching sheath/core filaments. It states that there is no particularlimitation of the mixture ratio of the two components; that a weightproportion of 40-70% of the lower melting component is preferred; andexemplifies 50-60%. It further teaches that the melt flow ratio of thepolypropylene component to the high density polyethylene componentshould not exceed 5.0 (see Chisso's Comparative Example 2 and claim 1).

Mitsubishi Rayon's Japanese Patent No. 50-4767 relates to a process formanufacturing bicomponent fibers for synthetic fiber paper. It teaches,inter alia, that paper can be made from chopped fibers comprisingheat-treated heterofilaments having a polypropylene core and apolyethylene sheath. It further teaches that low density polyethylene ismost suitable as compared with high density polyethylene and mediumdensity polyethylene. However, it does not include any actual examplesrelating to high density polyethylene, and all its examples relate toheterofilaments containing polyethylene having at least 50 percent ofthe cross-section of the filament being polyethylene (e.g. Table 1 andFIGS. 1A-1E).

Several references relate to nonwoven spunbonded film-fibril sheets ofsubstantially continuous plexifilamentary strands of high densitypolyethylene. U.S. Pat. No. 3,619,339 discloses a point-bonded producthaving around 1.3 oz/yd², tongue tear strength of 2.9 lb and tensilestrength of 50 lb/4 in. Similarly an article entitled "Spun bonded sheetproducts" by Hentschel in CHEMTECH, January, 1974, pages 32-41 givesdetailed properties of both area-bonded and point-bonded Tyvek® sheets.Sheets of weight 1.0-2.7 oz/yd² have Elmendorf tear strengths of 0.8-3.5lb, with area-bonded fabrics having Elmendorf tear strength of up to 1.1lb. "The Properties and Processing of TYVEK® Spunbonded Olefin" BulletinS10, published by du Pont in December, 1973 states at page 7 that "Whileit is possible to fuse TYVEK to itself using heat only, strong seals aredifficult to obtain in this way: melting the TYVEK destroys the fiberstructure, reducing both the flexibility and tear strength in the sealarea. The preferred method is to apply a coating whose melting point isbelow that of TYVEK, such as branched polyethylene or SURLYN A. Withsuch a coating, high seal strengths can be achieved using hot bar orimpulse techniques."

The foregoing prior art is shown diagrammatically in FIG. 1 with regardto the two parameters "amount of binder in binding filament" and "weightpercent of binding filament in fabric" (see definition of "bindingfilament" below). Other parameters such as type of heterofilament(sheath/core or side/side) and types of polymer in the heterofilamentsare given in the key to FIG. 1.

SUMMARY OF THE INVENTION

In contrast to the forementioned prior art, it has now been discoveredthat high density polyethylene, having a solid state density in therange from 0.930 to 0.965 gm/cc and having a melt flow index from 1 to50 as measured in ASTM D-1238, is startlingly superior to low densitypolyethylene when used as the sheath component in bonded fabricscomprising continuous sheath/core heterofilaments in which the core isisotactic polypropylene, provided that the high density polyethylenecomprises from 5 to 30 weight percent of the heterofilaments,particularly for essentially fibrous fabrics containing at least 70weight percent of heterofilaments. It has also been found that theproperties obtained with high density polyethylene are superior toexisting commercially available selfbonded heterofilament nonwovenfabrics. Further, processing conditions are relatively critical if thesesuperior properties are to be obtained consistently.

The products have utility as nonwoven fabrics, both industrial andnon-industrial, and include for example those uses listed in U.S. Pat.No. 3,341,394, column 28. The fabrics have particular utility for civilengineering applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 compares the prior art with the instant invention.

FIGS. 2-5 show enlarged cross-sectional views of typical sheath/coreheterofilaments before and after selfbonding.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In its broadest aspect, this invention relates to an improved selfbondednonwoven fabric having a weight of at least 40 gm/meter² and consistingessentially of fibrous nonparticulate material and comprising at leastseventy percent, based on total weight of fibers in the fabric, of drawncontinuous heterofilaments comprising at least two fiber-formingsynthetic polymer components arranged in sheath/core manner, the coresbeing isotactic polypropylene physically entrapped in a network ofbonded sheaths having a melting point at least 10° C. lower than themelting point of the core wherein the improvement comprises the use as asheath of high density polyethylene having a solid state density in therange from 0.930 to 0.965 gm/cc and having a melt flow index from 1 to50 as measured by ASTM D-1238, and the high density polyethylenecomprises from 5 to 30 weight percent of the heterofilaments; and thefabric has an Elmendorf tear strength (ASTM D-1424), T, of at least 6 lband a normalized grab tensile strength, G, of at least 120 lb (ASTMD-1117) for a 140 gm/meter² fabric and the value of the product TxG isat least 1,200. Such fabrics have greater normalized grab strength (asdefined below), fatigue resistance (as defined below), and tearresistance (as defined below), than prior art random nonwoven fabrics.Accordingly, such fabrics are outstandingly suitable for civilengineering applications such as road underlay material requiring highstrength and high fatigue resistance.

It is preferred that the value of the product TxG be at least 1800, andmost preferably at least 2,300.

It is preferred that the nonwoven fabrics be area-bonded rather thanpoint-bonded.

Homofilaments may be present in the fabrics of this invention in anamount of up to 30 percent by weight. It is preferred that thehomofilaments be prepared from the same polymer as the cores of theheterofilaments. It is preferred that the heterofilaments comprise atleast 98 percent by weight of fabric, and most preferably essentially100 percent (Example 5 c.f. Comparative Example 10).

It is preferred that the sheaths comprise 10 to 20 weight percent of theheterofilaments (Example 2 c.f. Example 4 and Comparative Example 8; andExample 5).

It is preferred that the high density polyethylene sheath has a meltflow index in the range 1 to 20. It may comprise up to 5 weight percentof a vinyl monomer, such as 1-hexene, 1-butene, etc. copolymerizedtherewith, provided that its solid state density is greater than 0.930gm/cc, and preferably greater than 0.940 gm/cc.

For applications involving filtration it is preferable for the fabric tohave a porosity (defined below) in excess of 70 percent. It has beenfound that the use of high density polyethylene sheath rather than lowdensity polyethylene sheath permits higher porosities to be obtained atequal tensile strength.

It is preferred that the filaments have deniers in the range from 10 to20.

It is important that the process used results in sheaths that aresubstantially concentric with their respective cores, otherwiseexcessive variability of fabric properties may be encountered.

Quench conditions affect the runnability of the process, butsurprisingly seem to have relatively little affect on the productproperties obtained (Example 2 c.f. Example 1).

The following definitions and test procedures are used throughout thespecification, unless otherwise stated.

"High density polyethylene" is a polymer or copolymer of ethylene whichhas a density greater than 0.930 grams/cc.

"Binding filament" is the type of filament which is selfbonded in aselfbonded nonwoven fabric. It may be a homofilament having a lowermelting point than the other filaments in the fabric or be aheterofilament having a lower melting component in at least part of itsouter surface.

"Melt flow index" (MFI) was measured by ASTM D-1238.

Polymer melting point was measured by differential thermal analysis(DTA) at the peak.

A vinyl comonomer is a monomer containing a single vinyl group, such ashexene-1 and butene-1.

Nonwoven fabrics were tested on a table top Instron testing machineusing ASTM D-1682 procedure for "grab tensile strength" and"elongation."

"Normalized grab strength" was obtained by multiplying the measured grabstrength in lb by 140 gm/meter² and dividing by the specimen wt/unitarea expressed in gm/meter². "Hand tearing" is a subjective test and wascarried out by cutting a 1 inch nick perpendicular to the edge of thefabric and attempting to propagate that cut by a fast shearing motion ofthe hands. Fabrics are typically easily hand tearable if the Elmendorftear is below 6 lb; torn with difficulty if the Elmendorf tear is 7-9lb; and torn with great difficulty or untearable if the Elmendorf tearis greater than 10 lb.

"Fatigue resistance" was carried out by a procedure similar to ASTMD-1682, except that the fabric was cycled between the limits of 0 to 120lb at the reduced rate of extension of 5 inches per minute until failureoccurred.

"Elmendorf tear strength" was measured by ASTM D-1424.

"Toughness" of a fabric is numerically equal to one two hundredth partof the product of the fabric's tenacity at failure and the fabric'spercent elongation at failure; and approximates the energy absorbingability of the fabric.

"Tensile factor" of a fabric is numerically equal to the product of thefabric's tenacity at failure and the square root of the fabric's percentelongation at failure.

"Porosity" of a fabric is a measure of the free space within a fabricand is expressed as a percentage. It can be calculated for example fromdata obtained by weighing a fabric of known filamentary composition andfilamentary specific gravity and measuring the thickness and area of thefabric.

A novel "test track" test was used to measure relative performance orcycles to failure of fabric under very severe wheel loading trafficconditions on a simulated unpaved road over subgrade of very low loadbearing capacity. It is described in detail below.

In summary, a sample of the fabric to be tested is clamped in a no slipcondition over a horizontal subgrade of predetermined load bearingcapacity and covered by an aggregate of predetermined gradation andcompaction. The unit is then cyclicly loaded vertically by reciprocatinga loaded trailer tire horizontally until failure of the fabric occurs.The test is monitored by an electronic recording system and deformationis recorded in chart form.

The following definitions are used in this test. "Test track" is themachine described below used to simulate traffic over temporary accessroads. "Stabilization performance" is the ability of the fabric towithstand cyclic loading until rupture occurs. "Deformation" is themeasured depth of rut at predetermined points.

The test track apparatus consists essentially of:

(A) A trough 18" broad×96" long×27" deep for supporting the subgrade,fabric and aggregate;

(B) A pressurized trailer tire (4 ply, 5.70/5.00-8) mounted on acarriage and at 50 psig tire pressure;

(C) A hydraulic pump for horizontal movement of the tire carriage;

(D) Pneumatic cylinders for vertical load application of wheels; and

(E) An electronic device for recording deformation and number of cycles.

The following outline procedure is used:

1. Prepare subgrade (fine grain soil) having a cohesive strength of 156lb/ft² measured by a vane shear device, by (i) adding water to GooseLake Fire Clay (from Illinois) until the concentration of water is 25percent by weight and (ii) mix in a mortar mixer until a soil cohesivestrength of 156 lb/ft² ±12 lb/ft² is obtained as measured by vane shear(ASTM D-2573).

2. Prepare Modified #7 Crusher Run aggregate by (1) obtaining standardN.C. Type ABC Crusher Run aggregate (approx. 700 lb); (2) removing allstones larger than 3/4" sieve; (3) air drying the aggregate for about 2days; (4) sieving aggregate to segregate fines than pass No. 100 sieve,and then restoring some of these fines to the aggregate in an amount of2% by weight of the aggregate; (5) adding Goose Lake Fire Clay in anamount of 2% by weight of aggregate; and (6) adjusting aggregatemoisture content to a level of 6-7% by weight as measured by aconventional Troxler Nuclear Moisture-Density Gauge.

3. Fill trough with the subgrade to a depth of 12 inches and level byhand tamping.

4. Place sample of fabric directly on subgrade and secure fabric on allsides so that the fabric is taut and in a no slip condition but nottensioned significantly. (This step is simplified by having the troughsplit horizontally at a depth of 12 inches).

5. Place 4 inches of aggregate on top of the fabric and compact to a wetdensity of 140 lb/ft³ at 7% moisture content by weight.

6. Set the tire pressure to 50 psi±1.0 psi.

7. Set the vertical wheel load, L, to 560 lb for soil cohesive strength,c, of 156 lb/ft². For variations in soil cohesive strength between theallowable values of 144 lb/ft² and 168 lb/ft²,

    L=3.59c

8. Run the test track at a linear horizontal velocity of 1 ft/sec in alaboratory at 70° F. and 50% relative humidity, and record the number ofcycles to failure, i.e. when subsoil extrudes through the aggregate. Inaddition, it is normal in the test to measure the deformation atpredetermined points at predetermined intervals; to recheck the shearstrength and moisture content of the subgrade at the end of the test;and to note the zone in which failure of the fabric occured.

The following Examples illustrate the invention and the preferredembodiments.

The Comparative Examples are not prior art, but show the surprisingnature of the invention. The invention is not limited to these Examples,which are merely given for purposes of illustration.

Examples 1-5 illustrate the invention. Examples 6-11 are ComparativeExamples.

Example 1

Hercules 6323 polypropylene (a high flow, maxiumum stiffness, fibergrade polymer having an MFI₂₃₀ of about 10 gm/10 min., and a solid statedensity of 0.903 gm/cc) and USI's LS506 high density polyethylene (witha density of 0.949 gm/cc, a Vicat softening point of 123° C. by ASTMD-1525, MFI₁₉₀ of 6 gm/10 min. by ASTM D-1238, DTA peak method meltingpoint of 127° C., and apparently a vinyl comonomer content of about 1/2percent) were spun in a core/sheath configuration through a 300 hole0.030" diameter 2:1 L/D scatter pattern spinneret at 17.15 lb/hr and 2.5lb/hr respectively.

The spun polypropylene had an MFI₂₃₀ of 20 gm/10 min. whilst the sheathpolyethylene had an MFI₁₉₀ of 7 gm/10 min. The pack was jacketed withDowtherm at 200° C. and the filaments were quenched by cross flow air inthe following manner. Two hundred cubic feet of air at ambienttemperature (about 70° F.) was blown each minute across the threadline.The quench unit consisted of fine mesh metal screens 2 feet long and 1foot broad arranged to give a velocity profile of 250 ft/min at adistance 21/2 inches below the spinneret and decreasing approximatelylinearly to 50 ft/min at a distance of 2 feet 21/2 inches below thespinneret. The filaments, containing 12.7% of high density polyethylenesheath, were taken around a heated roll (115° C.) at 1000 fpm and drawnto a draw ratio of 3.2 by taking several wraps around a heated draw roll(100° C.). The drawn filaments were fed to a traversing pneumatic spraygun, charged electronegatively and deposited as a uniform web on amoving conveyor.

The web was taken from the conveyor and passed through a drum type hotair bonding oven at 85 fpm. The web was restrained on the drum by a 50mesh steel belt exerting a pressure of 40 gm/cm² on the web whilst airat 145° C. was passed through it. On exiting from the oven the web waspassed through a calender with a nip load of 3 Kg/cm. One of the rollswas rubber covered whilst the other was smooth steel. The resultingfabric had the properties listed in Table 1.

EXAMPLES 2a AND 2b

Webs were made in a similar manner to Example 1 except that a 378 hole0.025" 10:1 L/D spinneret and outflow quench were used. Dowthermtemperature was set at 220° C. The pack throughput was adjusted to givea 7.5 dpf core of MFI₂₃₀ 10 gm/10 min. polypropylene with: (a) a 13%sheath and (b) an 18% sheath of MFI₁₉₀ 17 gm/10 min. high densitypolyethylene. The following outflow quench conditions were used. Onehundred and sixteen cubic feet of air at ambient temperature (about 70°F.) was blown radially outwards from a quench stick 20 inches long. Thetop of the quench stick was 1 inch below the spinneret and the quenchstick consisted of a 13/8 inch outer diameter perforated aluminum pipesurrounded by polyurethane foam 3/8 inch thick. The hole spacing in thealuminum pipe was adjusted to give a velocity profile increasingapproximately linearly from the top of the quench stick to a maximum atthe bottom of the quench stick. The threadline speed and draw ratio wasthe same as in Example 1 but the feed roll was heated to 110° C. whilstthe draw roll was at ambient temperature.

The webs were bonded at 33 fpm in a flatbed hot air oven, by passing airat 130° C. through the web and nipping the heated web with a load of 9Kg/cm in a rubber/steel calender of about one centimeter contact lengthin the machine direction. The bonded fabrics had the properties listedin Table 1. The runnability of the process was superior to therunnability of Example 1, apparently because of the different quenchconditions.

EXAMPLE 3

Using the same pack and quench system as in Example 2, web was made witha core MFI₂₃₀ of 22 gm/10 min. and a sheath MFI₁₉₀ of 9 gm/10 min. Thesheath level was set at 10.7%. On this occasion both feed and draw rollswere at ambient temperature. The web was bonded with 50 psig saturatedpressure steam by passing the web sandwiched between cotton beltsthrough pneumatic seals into and out of a 2 ft. long pressure chamber.The seal pressure was 60 psi and the belt speed was 22 fpm. Theresulting fabric had the properties listed in Table 1.

                  Table 1                                                         ______________________________________                                        PROPERTIES OF HIGH DENSITY SHEATHED FABRICS                                                                       Fatigue                                                                       Resis- Test                               Ex-            Grab     Elong-                                                                              Elmen-                                                                              tance  Track                              am-  Web Wt.   Strength ation dorf  cycles/                                                                              cycles/                            ple  gm/meter.sup.2                                                                          lb/4"    %     tear lb                                                                             failure                                                                              failure                            ______________________________________                                        1    140       211       61   13.7  --     1380                               2a   140       250       82   13.7  1000   1100                               2b   140       274      105    8.9  1000   1590                               3    156       152      119   15.8   849   1140                               ______________________________________                                    

EXAMPLE 4

Webs were made according to the process described in Example 1 exceptthat the sheath level was reduced to 7.0%. The polypropylene MFI₂₃₀ was23 gm/10 min. and the feed roll temperature was 90° C. After bonding bythe process described in Example 3 the fabric exhibited the followingproperties: grab strength of 126 lb; elongation of 112%; and Elmendorftear strength of about 15.8 lb.

EXAMPLE 5

Fabric was produced in the same manner as Example 1, except that thefilaments were about 9.5 d.p.f. rather than 5 d.p.f. and bonding waseffected with 50 psi steam pressure rather than hot air. The product hadthe following properties for sheath/core ratios in the range 13/87 to28/72: grab tensile strength of 178-191 lbs; and elongation of 80-120%.The hand tear strengths ranged from impossible to tear for 13/87 totearable for 28/72.

Examples 6 and 7 are Comparative Examples using low density polyethylenesheath.

EXAMPLE 6 (COMPARATIVE)

A 200 hole 0.015" spinneret was used with a 4/3 L/D. The polypropylenecore (ICI's PXC3924) was extruded with an MFI₂₃₀ of 19 gm/10 min., whilethe sheath polymer was low density polyethylene (0.923 gm/cc) (ICI'sAlkathene XRM40) which was extruded with an MFI₁₉₀ of 22 gm/10 min.

Pack throughput was 39.6 lb/hr at a spinning temperature of 265° C. andthe core/sheath ratio was 80/20. The filaments were cooled by an outflowquench system as in Example 2 and the filaments were pulled away fromthe pack at 1000 fpm. After drawing to a draw ratio of 4 with rolls atambient temperature, the filaments were sprayed to form a web as in theprevious examples. The web was bonded with 10 psi pressure steam as inExample 3 with the inlet and outlet seal pressure being 18 and 14 psirespectively. Properties of the fabric are listed in Table 2.

EXAMPLE 7 (COMPARATIVE)

Using the same pack and spinning conditions as in Example 6, thethroughput was adjusted to 31.4 lb/hr, the sheath level to 30% and thefeed roll and draw roll speeds to 1000 fpm and 4500 fpm respectively.The resulting web of 8.7 dpf filaments was bonded with 16 psig saturatedpressure steam with inlet and outlet seal pressures of 31 and 22 psigrespectively. The properties of the fabric are given in Table 2.

                  Table 2                                                         ______________________________________                                        PROPERTIES OF LOW DENSITY SHEATHED FABRICS                                                                        Fatigue                                                                       Resis- Test                               Ex-            Grab           Elmen-                                                                              tance  Track                              am-  Web Wt.   Strength Elong-                                                                              dorf  cycles/                                                                              cycles/                            ple  gm/meter.sup.2                                                                          lb/4"    ation tear lb                                                                             failure                                                                              failure                            ______________________________________                                        6    134       116      59    9.5   --     216                                7    142       207      76    5.3   678    246                                ______________________________________                                    

It will be particularly noticed that the fabrics of Comparative Examples6 and 7, as compared with the fabrics of the invention shown in Examples1-3, had much lower life (as measured by fatigue resistance and testtrack sycles to failure tests) and much lower toughness (which isproportional to the product of grab strength and elongation) and/or muchlower Elmendorf tear strength. It should also be remembered that theprior art teaches that, by changing bonding conditions such astemperature and pressure, and/or binder level, grab tensile strength canbe increased at the expense of tear strength.

EXAMPLE 8 (COMPARATIVE)

Hercules 6323 polypropylene and USI LS506 high density polyethylene,each polymer having the properties shown in Example 1, were spun in acore/sheath configuration through a 300 hole (scatter pattern)spinneret, with 0.025" hole diameter and 0.250" hole length, at 7.5lb/hr and 2.5 lb/hr, respectively. The core polypropylene had an MFI₂₃₀of 50 gm/10 min while the sheath polyethylene had an MFI₁₉₀ of 7 gm/10min. The filaments were quenched by cross-flow air as in Example 1,taken around a roll at 1500 ft/min. and drawn to a draw ratio of 2.5.The feed and draw rolls were at ambient temperature. This combination ofprocess variables resulted in a drawn denier per filament of 2.0.

The drawn filaments were fed to a traversing pneumatic spray gun,charged electronegatively, and deposited as a continuous filament web ona conveyor the speed of which was adjusted to give the desired weightper unit area. The web was taken from the conveyor and passed through asaturated steam bonding oven at 148° C. for 6 seconds. Steam pressurewas maintained in a chamber by air bags at 60 psig. Under theseconditions, the bonded nonwoven fabric had a weight of about 1 oz/yd²,and the following properties:

    ______________________________________                                        1" strip tensile test                                                                   ##STR1##                                                            Elmendorf                                                                              3.5 lb                                                               tear test                                                                     ______________________________________                                    

EXAMPLE 9 (COMPARATIVE)

Fabrics were made in a similar manner to Example 1 except that a 193hole 0.025" 10/1 L/D spinneret was used. The core polypropylenethroughput was 19.4 lb/hr whilst that of the sheath was 2.83 lb/hr. Bothcore and sheath polymers contained 2% carbon black supplied in the formof a 35% concentrate in a low density polyethylene carrier. The Dowthermtemperature was 220° C. whilst the feed and draw foll temperatures were100° C. and 110° C. respectively. The resulting 140 gm/meter² webs werebonded as in Example 3 and yielded a fabric with the followingproperties: grab strength of 112 lb and grab elongation of 106%. Whensubjected to 250, 2 hr. cycles of 1 hr. 40 mins. light, 20 mins. waterspray and light in a carbon arc Weatherometer (ASTM G23) the sampleretained 88% of its original strength, whilst a control sample withoutcarbon retained only 11% of its original strength. A similar fabriccontaining only 1% of carbon retained 100% of its strength in the sametest. This example shows that even small amounts of particulate materialintimately blended within the filaments adversely affects their originalstrength.

EXAMPLE 10 (COMPARATIVE)

Fabric was made in the same manner as Example 5 except that thefilaments were about 10 d.p.f. rather than 9 d.p.f. and the productconsisted of a blend of 50% heterofilaments with 50% polypropylenehomofilaments. The product had the following properties: grab tensilestrength of 132 lb and elongation of 56-77%. This fabric is not withinthe claimed composition, and shows the inferior properties obtained whenthe fabric contains less than 70 percent heterofilaments.

EXAMPLE 11 (COMPARATIVE)

Drawn filaments were prepared exactly as in Comparative Example 8.Thereafter the filaments were taken up on bobbins, placed on a creel,crimped to 10 crimps per linear inch and 25% crimp, and cut into 1.5inch staple. The staple fibers were then opened, carded, and layered ona rotating drum. The nonwoven fabric bonded under the conditionsemployed in Comparative Example 6 had a weight of about 1 oz/yd² and thefollowing properties:

    ______________________________________                                        1" strip tensile test                                                                   ##STR2##                                                            Elmendorf                                                                              1.8 lb                                                               tear test                                                                     ______________________________________                                    

This fabric is not within the claimed composition, and shows theinferior properties obtained when staple fiber is used rather thancontinuous filaments as in Comparative Example 8.

What we claim is:
 1. An improved selfbonded nonwoven fabric having aweight of at least 40 gm/meter² and consisting essentially of fibrousnon-particulate material and comprising at least seventy percent, basedon total weight of fibers in the fabric, of drawn continuousheterofilaments comprising at least two fiber-forming synthetic polymercomponents arranged in sheath/core manner, the core being isotacticpolypropylene physically entrapped in a network of bonded sheaths havinga melting point at least 10° C. lower than that of the core, wherein theimprovement comprises:said sheath is high density polyethylene having asolid state density in the range from 0.930 to 0.965 gm/cc and having amelt flow index from 1 to 50 as measured by ASTM D-1238; said highdensity polyethylene comprises from 5 to 30 weight percent of saidheterofilaments; and said fabric has an Elmendorf tear strength (ASTMD-1424), T, of at least 6 lb and a normalized grab tensile strength, G,of at least 120 lb (ASTM D-1117) for a 140 gm/meter² fabric and thevalue of the product TxG is at least 1,200.
 2. The fabric of claim 1wherein said sheaths comprise 10 to 20 weight percent of saidheterofilaments and TxG is at least 1,800.
 3. The fabric of claim 2wherein said heterofilaments comprise at least 98 percent by weight ofsaid fabric and TxG is at least 2,300.
 4. The fabric of claim 1 whereinsaid high density polyethylene has a solid state density of at least0.940 gm/cc.
 5. The fabric of claim 1 wherein said high densitypolyethylene has a melt flow index in the range from 1 to
 20. 6. Thefabric of claim 4 wherein said high density polyethylene has a melt flowindex in the range from 1 to
 20. 7. The fabric of claim 1 wherein saidfabric is area-selfbonded and has an Elmendorf tear strength of at least10 lb and a normalized grab tensile strength of at least 150 lb and anelongation of at least 50 percent at maximum stress.
 8. The fabric ofclaim 1 having a test track life, L, of at least 1,000 cycles to failurenormalized to a 140 gm/meter² fabric.
 9. The fabric of claim 8 wherein Lis at least
 1500. 10. The fabric of claim 1 having a weight up to 200gm/meter² ; and comprising filaments having deniers in the range from 1to 20.